Broad band circuits



May 30, 1967 N. T. I Avoo 3,323,073

BOARD BAND CIRCUITS Filed Nov. 8, 1963 /m/emor.' /Vormcm 7'. 0x/00,

H/'s Afforney United States Patent O 3,323,073 BROAD BAND CIRCUITS Norman T. Lavoo, Latham, NX., assignor to General Electric Company, a corporation of New York Filed Nov. 8, 1963. Ser No. 322,320 6 Claims. (Cl. 330-56) This invention relates to broad band circuits and particularly to broad band coupling circuits for vacuum tubes operated at high frequencies.

The advancing state of microwave technology makes desirable more and more bandwidth with high gain in microwave transmission systems and the like. Not only are wide bandwidths with high gain desirable, but minimum phase shift is also necessary. Miniature microwave triodes and tetrodes fuliill ,these requirements in amplifying tubes, particularly the minimum phase shift consideration, and have the additional advantage of high density cathode emission making possible relatively high power operation. However, the bandwidth and gain of the transmission system is usually limited by the bandwidth of the input and output devices which complete the vacuum tube circuit. It is a purpose of the present invention t provide a complete vacuum tube circuit of the above type having greater bandwidth than heretofore available.

In accordance with an embodiment of the present invention, a vacuum tube amplier is employed in conjunction with a one-half wave grid-plate output cavity. Inductive means separate the length of this cavity into separate portions resonant at spaced frequencies defining a cavity pass band, the output being taken at the end of the cavity remote from the vacuum tube. -Pass bands on the order of percent at one gigacycle with gains of 12 db are achieved in accordance with the present invention.

The subject matter which I regard as my invention is particularly pointed out and distinctly claimed in the concluding portion of this specification. The invention, however, both as to organization and method of operation, together with further objects and advantages thereof, may best be understood by reference to the following description taken in connection with the accompanying drawings wherein like reference characters refer to like elements and in which:

FIG. 1 is a cross-sectional view of the broad band amplier in accordance with the present invention,

FIG. 2 is an equivalent circuit of the output cavity portion of a broad band circuit in accordance with the present invention, and

FIG. 3 is gain versus frequency characteristic provided in accordance with the present invention.

Referring to FIG. l, a miniature disc seal vacuum tube 1 having ring-shaped terminals is provided with a coaxial output cavity 2 having a physical length of somewhat less than one-half wavelength at the operating freqency, here being on the order of one gigacycle. The tube is also provided with an input circuit 3. The disc seal tube is of the general type described and claimed in Beggs Patent 2,680,824, assigned to the asignee of the present invention. Specically, a General Electric type 7768 tube was employed. The tube includes an anode terminal 4, a grid ring 5, and a cathode ring or terminal 6, as well as filament connections 7 and 8. The tube is Conveniently supported at ring 9 joined to an annular support 10 for grounded grid operation. An annular capacitive mica plate 11 conveniently separates ring 9 from output cavity 2, so that a B+ connection may be made to the latter.

The output cavity 2 is formed from an outer conductor 12, and an inner conductor 13 joined to anode terminal 4 and each extending coaxially from tube 1. An annular 3,323,073 Patented May 30, 1967 extension 14 of the central conductor divides the cavity into two somewhat less .than one-quarter wave sections and is conductively coupled to the outer conductor of the cavity by means of a pair of posts which may take the form of screws 15 and 16 disposed 180 apart around the circumference of the coaxial cavity in this specific embodiment. These screws have reduced diameter portions 17 and 1S where they abut the extension 14, having a diameter on the order of mils. The screws or posts appear as inductance which is common to the two sections of the cavity, the value of this inductance depending on the number, diameters, and lengths of the posts. The said posts conduct anode heat from conductor 13. Capacitive screws 47 and 48 extend into the beginning of the cavity for tuning purposes.

An output coaxial line 20 including an inner comductor 21 and an outer conductor 22 receives energy from cavity 2. A flange 23 of outer conductor 22 is separated from the outer conductor of the cavity with intermediate ring 24 which is insulated from the cavity. Thus the outer conductors are capacitively coupled together. IRing 24 has a central aperture 25 of diameter somewhat smaller than the inside diameter of outer line or the cavity.

The central conductor 21 of the output line is also capacitively coupled to the central conductor 13 of the cavity. Also central conductor 21 is indented at 26 where it approaches cavity 2 to receive a spring-loaded metal block 27 attached to central conductor 13 by means of a spring member 28. Central conductor 21 is insulated from block 27 to form a capacitor therebetween, adjustable with a nylon bolt 29 extending through flange 23 and bearing upon block 27.

Capacitive loading at the output end of the cavity is provided principally by the capacitance between conductor 13 and the aperture 25 of ring 24. Also affecting the value of this capacity is the output coupling through the capacitance between block 27 and conductor 21. The capacitive loading is made conveniently adjust-able employing an extension or ring 19 slidable on conductor 13, and also by means of series of capacitive screws 49 and 50 protruding toward ring 19. The ring extends part way to the outer conductor 12 of the coaxial cavity.

It should be noted the output cavity 2, having a physical length less than one-half wavelength, is an electrical one-half wavelength including the capacitive load on either end thereof provi-ded respectively by the grid-anode capacitance of tube 1 and the terminating capacitance at the opposite end of inner conductor 13, this terminating capacitance occurring principally between the end of 13 and the annulus provided at 25. The one-half wavelength of the cavity is divided into two one-quarter wavelengths at posts 15 and 16.

An input circuit 3 functions to join input line 3i) to the grid connection 5 and the cathode connection 6 of tube 1. Input circuit 3 is one-half electrical Wavelength between grid ring 5 and the input line 30, the rst onequarter wave of which is effectively shortened by the grid-cathode capacitance of the tube. One purpose of the input circuit is to balance this capacitance. Filament connections 7 and 8 are preferably made at the location of a null point found to exist along the input circuit at the end of the lirst quarter-wave, and these connections are brought outside the circuit through feed-through bypass capacitors 31 and 32 for coupling to an appropriate filament supply source. The remaining one-quarter wavelength of the input circuit is a coaxial matching section having an inner conductor 33 and an outer conductor 34, the latter being joined to annular support 10. Bypass capacitors 31 and 32 extend through outer conductor 34. Central lindrcal recess 35, the edge of which contacts cathode connection 6. Recess 35 is deeper in the .vicinity of filament connection 8 and also is apertured to receive connection from the outside of feed-through bypass capacitor 32. The remaining filament connection 7 joins the enlarged portion of the central conductor whereby the filament connection from feed-through capacitor 31 may attach to the outside of the central conductor at this point.

Since a one-half wave of coaxial line exists in input circuit 3 between grid ring 5 and input line 30, the grid impedance should approximately appear at the extremity of the input circuit. At this point outer conductor 36 of the input line, having an end fiange 37, is insulated from the outer conductor 34 of the input circuit to provide capacity both to the outer and inner conductor of the input circuit. Likewise inner conductor 38 is insulated from inner conductor 33 of the input circuit by means of a gap 39 therebetween having a spacing on the order of 1/@2 of an inch. This spacing and the capacity t-hereof can be adjusted to achieve the proper match between the input line and the input circuit over a considerable range of frequencies inasmuch as the surge impedance of the input line is on the order of 50 ohms and likewise the input impedance of tube 1 is on the order of 50 ohms, the tube being operated in grounded grid manner. Shunt capacitive loading at the input line may be increased with an annular apertured member (not shown) Imounted between flange 37 and conductor 34, in a manner similar to ring 24 in the output circuit.

An alternative configuration is shown by dashed lines in FIG. 1. Here impedance matching between the null point and the input line is achieved by extending the enlarged section of inner conductor 33 (at the tube) toward the input line. At the input line conductor 33 is connected to conductor 3S rather than being insulated therefrom.

The output circuit is usually more important in determining the bandwidth of the stage. The tube output impedance is higher, but is likewise to be matched to an output line 20 similarly having an impedance on the order of 50 ohms. The output cavity 2 is therefore resonant. In operation the output circuit is doubly resonant at spaced frequencies defining a broad pass band. The output cavity may be viewed either as a cavity havin-g an electrical length of one-half wavelength, or as two onequarter electrical wavelength cavities.

FIG. 2 is a schematic diagram of a lumped constant circuit which may be taken as the approximate lumped constant equivalent of output cavity 2 and adjacent components. In FIG. 2, capacitor 40 is the grid-anode capacitance of tube 1; inductance 41 represents the first section of coaxial line between tube 1 and posts 15 and 16, shunt inductance 42 represents the mutual inductive effect of posts 15 and 16, and inductance 43 represents the portion of coaxial line between posts 15 and 16 and aperture 25. Aperture 25 (as well as screws 49 and 50 and extension 19) may be simulated by a capacitor 44 and may be viewed as balancing the grid-anode capacitance of the tube. Capacitor 45 is the electrical equivalent of the capacitance between the inner conductor of the cavity and the inner conductor of the output line, as well as the capacitance between the outer conductor of the cavity and the outer conductor of the output line for matching the output line to the cavity. Resistance 45 is the impedance of output line 20. The sections of coaxial line are represented by inductances 41 and 43 because the length of the line sections are slightly less than one-quarter wavelength physically. Mutual inductance 42 arises because of the magnetic fields around posts 15 and 16 linking the two line sections. The first section of this circuit is tuned to resonance by shorting capacitor 44, and then tuning capacitor 40 to resonate the first portion of the cavity at the operating frequency. This may be accomplished physically in FIG. 1 by adjusting capacitive screws 47 and 48. Then, the tube end is shorted out by a number of conductors between grid and plate and the capacitor i 44 is tuned to resonance at the same frequency, for 1nstance, by means of screws 49 and 50 or by moving ring 19 longitudinally on 13. The output capacitance is also adjusted by varying the size of the aperture 25 or by varying the amount of insulation between ring 24 and the outer conductor of cavity 2. Adjustment of ring 24 is a gross adjustment and once nearly correct matching is achieved, the screws 49 and 50 and ring extension 19 will be sufficient in their effect for the purpose of tuning the second portion of the output cavity. Now, the connections between the tubes grid and plate may be removed. Although both portions of the cavity are tuned to the central frequency of the cavity, it will be found that these respective resonances tend to move apart because of the effect of the two cavity portions upon one another. lt is found the above procedure achieves approximately a l0 percent bandwidth at one gigacycle between the 1 db points as illustrated in the characteristic curve of FIG. 3.

The bandwidth is controlled by the value of the common inductance 42 achieved with posts 15 and 16, i.e. by the size and number of such rods employed. The cavity may possibly be viewed in the alternative as a complete `one-half wave resonator having the electrical field distribution shown in FIG. 1 opposite the output cavity where` in the curve 51 represents the -lhalf wave mode or usual half wave mode, i.e. the high frequency mode in the present circuit. The inductive perturbation'caused by posts 15 and 16 has little effect on this mode because there is substantially zero electric field across their inductance. On the other hand, at the lower frequency resonance, attributable to +4- mode, 52, in FIG. 1, the resonance is appreciably affected by the inductive posts. Thus for proper mode separation, and at the same time to provide a reasonably flat response in the FIG. 3 characteristic, the common inductance is increased to that value that lowers the -j--jmode to the correct value to achieve the desired separation between modes as illustrated by the respective peaks in the FIG. 3 characteristic. The appropriate or permissible dip in the center of the characteristic is also controllable by the capacitive coupling to the load. This loading is affected by the aperture and position of rin-g 24 and by the adjustment of the capacity between block 27 and conductor 21 as adjusted with nylon bolt 29.

While I have shown and described particular embodiments of my invention, it will be apparent to those skilled in the art that many changes and modifications may be made without departing from my invention in its broader aspects; and I therefore intend the appended claims to cover all such changes and modifications as fall within the true spirit and scope of my invention.

What I claim as new and desire to secure `by Letters Patent of the United States is:

1. A broad band circuit comprising a coaxial cavity having a length on the order of one-half wavelength at the operating frequency, a vacuum tube positioned at one end of said coaxial cavity having its anode coupled t0 the central conductor of said cavity and its grid coupled to the outer conductor of said cavity, inductive means separating the cavity into two separate resonant portions along the length of said cavity effective to resonate at spaced frequenciesdefining a pass band, and an end loading capacity between the inner conductor of said cavity and the outer conductor of said cavity at the end of said cavity remote from said vacuum tube.

2. A broadband circuit comprising a coaxial cavity having a length on the order of one-half wavelength at the operating frequency, a vacuum tube positioned at one end of said coaxial cavity having its anode coupled to the central conductor of said cavity and its grid coupled to the outer conductor of said cavity, inductive means separating the cavity into two separate resonant portions along the length of said cavity effective to provide resonance at spaced frequencies defining a pass band, said inductive means comprising at least one post extending from the louter conductor to the inner conductor of said coaxial cavity at a distance physically less than one-fourth wavelength from the grid of said vacuum tube but at a distance of substantially one-fourth electrical wavelength including the grid-anode capacitive loading presented by said tube, and an end loading capacity at the end of said cavity remote from said vacuum tube, said end loading capacitor being located at a distance physically somewhat less than one-fourth wavelength from said inductive means but approximately one-fourth electrical wavelength including the effect of said end loading Capacity.

3. A broad band circuit according to claim 3 wherein said end loading capacity includes a partial extension positioned between said inner conductor and said outer conductor of said coaxial cavity.

4. A broad band circuit comprising a vacuum tube including an `anode terminal, a cathode terminal, and a grid ring therebetween, a coaxial cavity coaxially aligned with said tube having a Wavelength on the order of one-half wavelength `at the opeerating frequency of said circuit, said coaxial cavity including a central conductor joined to said anode terminal and an outer conductor coupled to said grid ring, a pair of inductive posts separating the said cavity into two separate portions of slightly less than onefourth wavelength in physical length but of substantially one-fourth electrical wavelength, an output line, a member including an aperture between said output line and said cavity, said member including an aperture contributing capacitive end loading to said cavity, an output line, and capacitive means coupling said line to the portion of said cavity remote from said tube.

5. A broad band circuit comprising a vacuum tube including an anode terminal, a cathode terminal, and a grid ring therebetween, -a coaxial cavity coaxially aligned with said tube having a wavelength on the order of onehalf wavelength at the operating frequency, said anode terminal being joined to the central conductor of said coaxial cavity with said grid ring coupled to the outer conductor `of said cavity, a pair of inductive posts separating the cavity into two separate portions of slightly less than one-quarter wavelength in physical length but of substantially one-quarter electrical wavelength, means to provide capacitive end loading thereof, an output line,

capacitive means coupling said line to the portion of said cavity remote from said vacuum tube, an input line, and a one-half electrical wavelength coaxial input circuit extending coaxially from the remaining end of said tube including a one-quarter wavelength coaxial matching section forming the portion of said input circuit remote from said tube for coupling to said input line, said input circuit having an inner conductor coupled to said cathode and a coaxial outer conductor coupled to said grid ring, said matching section having an inner conductor of larger diameter than said input line for matching said input line to said tube.

6. A `broad band circuit comprising a vacuum tube i11- cluding an anode terminal, a cathode terminal, and a grid ring therebetween, a coaxial cavity coaxially aligned with said tube having a wavelength on the order of one-half Wavelength at the operating frequency, said anode terminal being joined to the central conductor of said coaxial cavity with said grid ring coupled to the outer conductor of said cavity, a pair of inductive posts separating the cavity into two separate portions of slightly less than one-quarter wavelength in physical length but of substantially one-quarter electrical wavelength, means providing capacitive end loading to said cavity, an output line and capacitive means coupling said line to the portion ot' said cavity remote from said vacuum tube, a coaxial input line, and a one-half electrical wavelength input circuit extending coaxially from the remaining end of said tube having .an inner conductor coupled to said cathode and a coaxial outer conductor coupled to said grid ring, said input line including a central conductor spaced from the inner conductor of said input circuit dening a gap therebetween for providing a matching capacitance between said input line and said input circuit.

References Cited UNITED STATES PATENTS 7/1963 Moulton 330-56 X 6/1964 Bridges et al. 330-56 X 

1. A BROAD BAND CIRCUIT COMPRISING A COAXIAL CAVITY HAVING A LENGTH ON THE ORDER OF ONE-HALF WAVELENGTH AT THE OPERATING FREQUENCY, A VACUUM TUBE POSITIONED AT ONE END OF SAID COAXIAL CAVITY HAVING ITS ANODE COUPLED TO THE CENTRAL CONDUCTOR OF SAID CAVITY AND ITS GRID COUPLED TO THE OUTER CONDUCTOR OF SAID CAVITY, INDUCTIVE MEANS SEPARATING THE CAVITY INTO TWO SEPARATE RESONANT PORTIONS ALONG THE LENGTH OF SAID CAVITY EFFECTIVE TO RESONATE AT SPACED FREQUENCIES DEFINING A PASS BAND, AND AN END LOADING CAPACITY BETWEEN THE INNER CONDUCTOR OF SAID CAVITY AND THE OUTER CONDUCTOR OF SAID CAVITY AT THE END OF SAID CAVITY REMOTE FROM SAID VACUUM TUBE. 